1. Field of the Invention
The present invention relates to a shift control device for an automatic transmission including frictional engaging elements such as shifting clutches for switching among power transmission paths composed of a plurality of shift gear trains to automatically perform shifting.
2. Description of the Related Art
Conventionally widely used is an automatic transmission in a vehicle such that frictional engaging elements such as shifting clutches are selectively engaged to thereby switch among power transmission paths composed of a plurality of shift gear trains, thus automatically performing shifting. In recent years, there is a trend that a shifting time of an automatic transmission (an engaging operation time of each frictional engaging element) has been increased for the purposes of higher power of an engine and improvement in shift quality of the automatic transmission, for example. In association with this trend, the amount of heat generated from the frictional engaging element subjected to shifting tends to increase in the automatic transmission.
In this case, the frictional engaging element heated to high temperatures in the shift operation is cooled by heat exchange or the like with a hydraulic fluid or automatic transmission fluid (ATF) for the automatic transmission, so that the temperature of the frictional engaging element lowers to a temperature substantially equal to the temperature of the ATF. Accordingly, suppression of a rise in temperature of the ATF is an important factor in protecting the facing of each frictional engaging element from thermal damage such as burning. For example, Japanese Patent Laid-open No. Hei 10-169483 discloses a technique of detecting the temperature of the ATF and continuously reducing the output torque from an engine when the ATF temperature detected above is greater than or equal to a predetermined temperature.
However, until the heat generated from each frictional engaging element is absorbed to the ATF by heat exchange to cool each frictional engaging element, a predetermined radiation time is required for the ATF and there remains a given difference in temperature between each frictional engaging element and the ATF until the radiation from the ATF is completed. Accordingly, in the case of simply suppressing a rise in temperature of the ATF as by the technique disclosed in Japanese Patent Laid-open No. Hei 10-169483 mentioned above, the measures against thermal damage to each frictional engaging element may be insufficient in some case. In such a particular case that the vehicle runs on a winding road, shift control may be repeatedly performed and the same frictional engaging element may be frequently engaged in a short period of time. In this case, there is a possibility that a large amount of heat may be stored in the same frictional engaging element even when the ATF temperature is low, causing thermal damage such as burning to the facing.
A shift control device for an automatic transmission which can solve this problem is proposed in Japanese Patent Laid-open No. 2005-98431, wherein frequently performed shift control can be supported to protect each frictional engaging element from thermal damage. The shift control device disclosed in this publication performs control so that when the amount of heat stored in each frictional engaging element exceeds an allowable value, the shift timing is changed to an earlier-than-usual timing to thereby reduce differential rotation of the disks, thereby suppressing the amount of heat generated to protect each frictional engaging element from thermal damage. In the shift control device disclosed in Japanese Patent Laid-open No. 2005-98431 mentioned above, the upshift timing is set to an earlier-than-usual timing, causing a problem such that a drive force desired by an operator cannot be sufficiently transmitted. Further, in particular shifting with a large amount of heat generated, such as shifting with partial return of an accelerator pedal, an enough satisfactory effect cannot be obtained.